sys_util: Convert mmap::Error over to thiserror::Error.
sys_util: Convert mmap::Error over to thiserror::Error.

This conversion adds source attribution for wrapped errors and
implicitly implements std::error::Error for mmap::Error which was
missing before.

Also some compile time warnings were fixed for the mmap::arena_msync
test case.

BUG=None
TEST=cargo test

Change-Id: I13b6257692887bcd7229f1cb59c10edb850d0a65
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3207106
Auto-Submit: Allen Webb <880184b418addffb87cd9c8e8e882ca6764ac96b@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Allen Webb <880184b418addffb87cd9c8e8e882ca6764ac96b@google.com>
Reviewed-by: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm